Examples of currently commercialized secondary cells include nickel-cadmium cells, nickel-metal hydride cells, nickel-zinc cells, and lithium secondary cells. Among such various secondary cells, lithium secondary cells are freely rechargeable because of having substantially no memory effect compared with nickel-based secondary cells, and have a very low self-discharge rate and high energy density. Owing to these merits, there has been high interest in lithium secondary cells.
In general, lithium secondary cells use a lithium-based oxide as a positive electrode active material and a carbonaceous material as a negative electrode active material. A lithium secondary cell may include: an electrode assembly in which a positive electrode plate coated with such a positive electrode active material and a negative electrode plate coated with such a negative electrode active material are disposed with a separator therebetween; and a case in which the electrode assembly and an electrolytic solution are sealed.
In general, according to case types, lithium secondary cells may be classified into a can type in which an electrode assembly is accommodated in a metal can, and a pouch type in which an electrode assembly is accommodated in a pouch formed of an aluminum laminate sheet.
In recent years, secondary cells have been widely used not only in small-sized devices such as portable electronic devices, but also in medium to large-sized devices such as automobiles or power storage devices. For use in such medium to large-sized devices, a large number of secondary cells may be electrically connected to increase capacity and output power. In particular, since it is easy to stack pouch-type secondary cells, pouch-type cells are widely used in such middle to large-sized devices.
The mechanical strength, that is, rigidity of a battery module including a plurality of pouch-type secondary cells as described above should be equal to or greater than a certain value so that the battery module may not be broken by external impacts, vibrations, or the like. In addition, it is necessary to securely seal battery modules to prevent permeation of foreign substances such as moisture. Moreover, battery modules having high assemblability and fewer components are preferable to increasing productivity.